This disclosure relates to rotor and stator assemblies that utilize magnetic bearings and can be used in corrosive environments and processes of assembling the magnetic bearings. The rotor and stator assemblies can be used in turboexpanders, pumps, compressors, electric motors and generators, and similar turbo-machinery for the oil and gas industry.
A turboexpander is an apparatus that reduces the pressure of a feed gas stream. In so doing, useful work may be extracted during the pressure reduction. Furthermore, an effluent stream may also be produced from the turboexpander. This effluent stream may then be passed through a separator or a distillation column to separate the effluent into a heavy liquid stream. Turboexpanders utilize rotating equipment, which is relatively expensive and typically includes a radial inflow turbine rotor mounted within a housing having a radial inlet and an axial outlet. The turbine rotor is rotatably mounted within bearings through a shaft fixed to the rotor. Such turboexpanders may be used with a wide variety of different gas streams for such things as air separation, natural gas processing and transmission, recovery of pressure letdown energy from an expansion process, thermal energy recovery from the waste heat of associated processes, and the like. Compressors can be associated with turboexpanders as a means to derive work or simply dissipate energy from the turboexpander.
There are three primary types of bearings that may be used to support the rotor shaft in turbomachinery such as the turboexpander or compressor noted above. The various types of bearings include magnetic bearings, roller-element bearings, and fluid-film bearings. A magnetic bearing positions and supports a moving shaft using electromagnetic forces. The shaft may be spinning (rotation) or reciprocating (linear translation). In contrast, fluid-film and roller-element bearings are in direct contact with the rotor shaft and typically require a fluid based lubricant, such as oil.
Magnetic bearings provide superior performance over fluid film bearings and roller-element bearings. Magnetic bearings generally have lower drag losses, higher stiffness and damping properties, and moderate load capacity. In addition, unlike other types of bearings, magnetic bearings do not require lubrication, thus eliminating oil, valves, pumps, filters, coolers, and the like, that add complexity and includes the risk of process contamination.
In a typical magnetic bearing arrangement for rotor and stator assemblies, a stator comprising a plurality of electromagnetic coils surrounds a rotor shaft formed of a ferromagnetic material. Each of the electromagnetic coils, referred to as magnetic radial bearings because they radially surround the rotor, produce a magnetic field that tends to attract the rotor shaft. The rotor shaft assembly is supported by these active magnetic radial bearings inside the stator at appropriate positions about the rotor shaft. By varying the amount of current in the coils of a particular magnet, the attractive forces may be controlled so that the rotor remains centered between the magnets. Sensors in the stator surround the rotor and measure the deviation of the rotor from the centered position. A digital processor uses the signals from the sensors to determine how to adjust the currents in the magnets to center the rotor between the magnets. The cycle of detecting the shaft position, processing the data, and adjusting the currents in the coils, can occur at a rate of up to 25,000 times per second. Because the rotor “floats” in space without contact with the magnets, there is no need for lubrication of any kind.
Anti-friction bearings as well as seals may be installed at each end of the rotor shaft to support the shaft when the magnetic bearings are not energized. This avoids any contact between the rotor shaft and the stator's radial magnetic bearings. These auxiliary or “back-up” bearings are generally dry, lubricated, and remain unloaded during normal operation.
In the oil and gas industry, the rotor and stator assemblies can operate in a process gas, which can also serve as a cooling agent. The process gas typically is natural gas at pressures of about 10 bar to about 200 bar. Unfortunately, natural gas can have a high degree of contaminants. These contaminants can include corrosive agents such as hydrogen sulfide (H2S), water, CO2, oil, and others. In the worst case, the combination of water and H2S leads to what is called wet sour gas, a more corrosive gas. Magnetic bearings typically require cooling so as to maintain an acceptable temperature in the bearing components. Utilizing the process gas directly as the coolant provides a significant advantage in enabling a seal-less system, which eliminates the need for buffer gases (which are not generally available in upstream oil and gas applications) and enhancing safety and operability of the turbo-machinery installed. However, the cooling of the magnetic bearing assembly, and hence its use, in a process gas environment that contains the above contaminants poses a significant risk to the vulnerable components of the magnetic bearing.
The National Association of Corrosion Engineers (NACE) Standard MR0175, “Sulfide Stress Corrosion Cracking Resistant Metallic Materials for Oil Field Equipment” is a widely used standard in the oil and gas industry that specifies the proper materials, heat treat conditions, and hardness levels required to provide good service life of machinery used in sour gas environments. A NACE compliant material or component is substantially resistant to corrosion such as may occur upon exposure of a non-NACE compliant material to sour gas and/or wet sour gas. For example, NACE compliant welds generally require a post-weld heat treatment process to relieve any weld stresses that would normally contribute to the susceptibility for corrosion. Currently, there are no magnetic bearing systems used in the oil and gas industry that are fully NACE compliant.
NACE compliance is desirable because the rotor shaft assembly includes several components that could be exposed to a sour gas environment during operation. These include, among others, the rotor shaft itself, the magnetic rotor laminations about the rotor shaft, and the rotor-landing sleeves. As an example of the sensitivity to corrosive agents, it has been found that if the rotor laminations are exposed to wet sour gas they typically fail due to hydrogen embrittlement and stress-related corrosion cracking. Stress related corrosion cracking is an issue since the magnetic rotor laminations are typically manufactured as punchings that are shrunk-fit onto the rotor shaft. During operation at working speeds, these components experience relatively high mechanical stresses due to the shrink-fit stresses and radial forces imparted thereon.
Another drawback of current magnetic bearing systems used in rotor and stator assemblies relates to the steel alloys typically used in the construction of the rotor shaft and/or rotor laminations. The selection of steel compositions that are most resistant to sour gas generally have poor magnetic properties. Because of this, high electromagnetic losses on the rotor shaft occur resulting in heat loads exceeding 1.00 W/cm2 (6.45 W/in2). The exposure to the high temperatures from the heat loads can lower resistance of the steels to sour gas corrosion. Increasing the size of the components to minimize the heat loads is not practical in view of the costs, and foot prints associated with the larger components.
In addition to the rotor shaft and laminations, the rotor shaft assembly typically includes a rotor landing sleeve shrunk-fit onto each end of the rotor shaft. This landing sleeve engages an inner race of a roller-element backup bearing in the event of a rotor landing, during which the magnetic bearing fails and the backup bearing has to support the rotor during the subsequent shut-down procedure. Currently, the rotor landing sleeve is formed of a material that is not NACE compliant and is therefore subject to corrosion in a sour gas environment.
The magnetic bearing stator is a stationary component that provides the source of the magnetic field for levitating the rotor assembly. An air gap separates the stator from the rotor shaft. In order to maximize the magnetic field strength and the levitation force this air gap is made as small as possible while still meeting mechanical clearance requirements between the rotor shaft and the stator. The gap size is typically on the order of millimeter fractions. If the gap is increased, the coils in the stator require more current to levitate the rotor, or the diameter or axial length of the stator has to be increased, all of which increase the overall stator size. If the stator size is limited and cannot be increased, then the levitation force is reduced if the air gap is larger than required by mechanical clearances.
Current stators are either encapsulated or non-encapsulated. In the case of encapsulated stators, a stator “can” protects the stator components from the process environment. Current stator cans are generally comprised of two concentric tubes of the same material joined at the ends. This tubular can section is located in the gap between the stator and the rotor shaft. If the can material is non-magnetic then it adds an additional magnetic gap on top of the required mechanical clearance, which reduces bearing capacity. In order to maintain bearing capacity, the material of the tubular can section can be selected to be magnetic.
In current practice, the stator can sections are assembled from magnetic NACE compliant alloys (typical examples are chromium-nickel alloys with a 15-18 wt % chromium 3-5 wt % nickel and 3-5 wt % copper content such as 17-4 precipitation hardened (PH) stainless steel) and are welded together. The welds would normally require a post-weld heat treatment at temperatures in excess of 600° C. in order to be fully NACE compliant. However, due to the temperature limits of the encapsulated electric stator components and the method of current manufacture, no heat treatment is possible. Therefore, the welds are not currently NACE compliant and are subject to corrosion and failure such as from exposure to sour gas. Moreover, some components of the stator, such as sensors, as well as power and instrumentation wires, cannot be encapsulated and are exposed to the process gas environment.
Referring now to prior art FIG. 1, there is shown an exemplary turbo expander-compressor system generally designated by reference numeral 10 that includes a rotor and stator assembly having multiple magnetic bearings for supporting a rotor shaft. The system 10 includes a turbo expander 12 and compressor 14 at opposite ends of a housing 16 that encloses multiple magnetic bearings 18 for supporting rotor shaft 20.
Each magnetic bearing 18 includes a stator 22 disposed about the rotor shaft 20. The stator 22 includes stator poles, stator laminations, stator windings (not shown) arranged to provide the magnetic field. Fixed on the rotor shaft 20 are rotor laminations 24, each rotor lamination aligned with and disposed in magnetic communication with each stator 22. When appropriately energized, the stator 22 is effective to attract the rotor lamination 24 so as to provide levitation and radial placement of the rotor shaft 20. The illustrated system 10 further includes additional axial magnetic bearings 26 and 28 so as to align the rotor shaft 20 in an axial direction by acting against a magnetic rotor thrust disk 30. Roller-element backup bearings 32 are disposed at about each end of the rotor shaft and positioned to engage a rotor landing sleeve 34 disposed on the rotor shaft 16 when the magnetic bearings fail or when system 10 is in an off state. When the system 10 is configured to accommodate axial or thrust loads, the width of the sleeve 34 is increased to accommodate any axial movement.
The backup bearings 32 are typically made of roller-element bearings. In such bearings, the inner and outer races require steel alloys of high hardness (typically in excess of HRC 40 (Rockwell C-Scale Hardness)) to accomplish low wear and long bearing life. However, in steel alloys, the properties of high hardness and corrosion resistance are contradicting requirements. As a result, current races are made of high-hardness steel alloys that do not meet NACE corrosion requirements.
The system 10 further includes a plurality of sensors represented by 36 as well as power and instrumentation wires 38 in electrical communication with controller units (not shown). The sensors 36 are typically employed to sense the axial and radial discontinuities on the rotor shaft 20 such that radial and axial displacement along the shaft can be monitored via the controller unit so as to produce a desirable magnetic levitation force on the rotor shaft 20.
Prior art FIG. 2 illustrates a partial cross-sectional view of an exemplary rotor and stator assembly 50. The rotor and stator assembly 50 includes a rotor shaft assembly 52 that includes rotor laminations 54 attached to a rotor shaft 56. An encapsulated stator assembly 60 surrounds the rotor shaft assembly 50 and includes a stator frame 62, magnetic stator laminations 64 wrapped in conductive windings 66, and a stator sleeve 68. The stator sleeve 68 generally has a thickness ranging from 0.05 to 5.0 millimeters (mm). The encapsulated stator assembly 60 includes a hermetically sealed can defined by walls 70 and the stator sleeve 68, the walls 70 having a thickness of about one centimeter. The can is formed from multiple sections that are welded at various interfaces 72. These welds are not NACE compliant. Other stator components not shown are stator slots, poles, sensors, and power and instrumentation wires. An air gap 80 separates the rotor shaft assembly 52 from the stator assembly 60. In operation, the rotor shaft 56 levitates in a magnetic field produced by the stator assembly 60.
Given the increasing use of rotor and stator assembly that utilize magnetic bearing systems in corrosive environments, a growing need exists to overcome the above-described deficiencies of current magnetic bearings.